Luminescent phosphor screens are used in cathode ray tubes, for example, television display tubes, electron display devices, imaging devices, for example, image intensifier tubes, etc. Typically, a thin layer of phosphor material containing a luminescence activator is supported on a substrate. The phosphor layer is activated by impingement of an electron beam, and the resulting luminescence is transmitted through the glass substrate at the front of the display. The phosphor layer may be formed as a monocrystalline layer grown on a substrate by liquid phase epitaxy (LPE), or as a thin film deposited by evaporation, sputtering, or vapor deposition (MOCVD/MOVPE) techniques. Such phosphor layers have a relatively high thermal loadability and luminescence.
However, due to a difference in index of refraction, most of the light that is generated by the electron beam in the phosphor layer is internally trapped by reflection from the substrate layer, resulting in a relatively low external screen efficiency. Other types of phosphor layers, such as powdered phosphors, may be used to avoid the reflection losses, but these have comparatively low thermal loadability, low resolving power, and/or high outgassing losses in the vacuum manufacture of a cathode ray tube. By comparison, a thin film phosphor screen has a high resolution and a low outgassing characteristic which enhance its life and performance and make it particularly suitable for devices such as image intensifier tubes.
The problem of internal reflection of monocrystalline or thin film phosphors is illustrated schematically in FIG. 1. An electron beam e.sup.- impinges on the phosphor layer through a metal layer, e.g. aluminum, which is optional in some applications. The electron beam activates an activator element, for example, copper in zinc-sulfide based phosphors, or cerium in yttrium-aluminum-garnet phosphors, which causes electrons to be released and photons from the nearby phosphor material to be emitted with a luminescence effect. Due to the difference in index of refraction between the phosphor layer and the substrate layer, such as glass, light rays which are incident at an angle greater than the critical angle CA are reflected internally and become trapped and dissipated within the film. Another form of light loss is attributable to reflections from the substrate layer even within the cone (indicated by the dashed lines) of the critical angle CA, which increases as the light rays approach the critical angle.
As an example, the internal reflection loss due to the refraction difference for ZnS based phosphors grown on Corning type 7056 glass substrate can be as high as 75% to 80% of the light emitted. Within the acceptance angle, the reflection loss can be another 10, for a total loss of about 90% of the radiated energy. Such high losses result in lower phosphor efficiencies than other types of phosphor layers, e.g. powdered phosphors. The result is that thin film phosphors have had limited application heretofore.
Some researchers have proposed forming reticulated structures in the phosphor layer to break up the waveguide effect and enhance light output. For example, U.S. Pat. No. 4,298,820 to Bongers et al. discloses the technique of etching V-shaped grooves in square patterns in the surface of the phosphor layer to obtain improved phosphor efficiency by a factor of 1.5. However, the etching process used in Bongers has been found to be impractical for large volume production.
Etching the activated portion of the phosphor layer with reticulations in the form of trapezoid- or truncated-cone-shaped mesas and overcoating with a reflective aluminum film to form light confining surfaces has been proposed in the article entitled "Reticulated Single-Crystal Luminescent Screen", by D. T. C. Huo and T. W. Hou, Journal of Electrochemical Society, Vol. 133, No. 7, pp. 1492-97July 1986, and in "RF Sputtered Luminescent Rare Earth Oxysulfide Films", by Maple and Buchanan, Journal of Vacuum Technology, Vol. 10, No. 5, pg. 619, September/October 1973. These trapezoidal mesas improve the light output by a factor of about 2, whereas a factor of 6 or higher would represent output of most of the emitted light. The light output factor could be increased if the mesa size could be made less than 5 microns and the shape made with the optimum reflection angle. However, such a small mesa size requires high lithography resolution and is limited by diffraction from the lithography mask. Crystalline phosphors will also preferentially etch along crystalline planes which are different from the optimum slope angle for the trapezoid shape. Thus, application of trapezoidal mesas has also been limited.